Conventionally, four-wheel-drive (4WD) vehicles are put to practical use, which travel by driving front wheels (main drive wheels) and are provided with a rear-wheel-drive system having a coupling part capable of distributing a driving force to rear wheels (part-time drive wheels) when a slip occurrence of the front wheels is detected during the travel. This allows the structure of the rear-wheel-drive system to be lighter in weight compared to a vehicle which always travels in a 4WD mode, and prevents, by normally traveling in a front-wheel-drive mode, a loss of the driving force in the rear-wheel-drive system, which are advantageous in achieving improvements in both traveling performance on a road surface where the slip easily occurs and fuel efficiency.
For example, JP5793877B discloses a control device for such a 4WD vehicle which distributes a driving force to rear wheels via a coupling part and reduces a loss of the driving force when a front-wheel slip amount detected based on a vehicle speed and a front wheel speed increases, which is when the loss of the driving force increases.
Meanwhile, when the vehicle turns, a ground contact load of an inner wheel of drive wheels decreases and a ground contact load of an outer wheel of the drive wheels increases due to rolling of the vehicle. It was found, from measuring ground contact load of the wheels by causing the vehicle to actually travel on a test road applying a general traveling pattern, that the ground contact loads of each wheel changes by about 10%-50% when turning. Since a reduction in the ground contact load lowers a force of friction with a road surface, a total force of the driving force of the inner wheel and a lateral force acting thereon easily exceeds the frictional force when turning, which facilitates the occurrence of slip. The present applicants have already proposed an art for predicting an occurrence of such a slip and preventing it (JP2016-228188).
On the other hand, it was found from the measurement of the ground contact loads that, even on a smooth pavement surface, each ground contact load changes by about 10% at most due to an undulation of the road surface, etc., and on a rough pavement surface with uneven parts because of cracks, etc., the ground contact load changes by about 15% at most due to the unevenness of the road surface in addition to the undulation thereof. Therefore, on the rough road surface, since the ground contact load decreases more than on the smooth road surface, the possibility of slip occurrence increases. Particularly when the vehicle turns, the ground contact load of the inner wheel decreases due to the rolling as described above, and therefore, the total force of the driving force and the lateral force exceeds the frictional force more easily and the slip easily occurs.
However, no art is known for controlling a driving force corresponding to such a rough road surface, and the slip caused by the rough road surface cannot be prevented. If the driving force is to be distributed to part-time drive wheels sufficiently before the total force of the driving force and the lateral force exceeds the frictional force in order to prevent the slip caused by the rough road surface, a frequency of driving the part-time drive wheels increases and fuel efficiency may degrade.